Apparatus for welding



OCt- 14,1947- n F. H. JOHNSON ETAL l 2,429,185

APPARATUS FOR WELDING Filed April :5, 1943 s sheets-sheet 1 Slg E.;

ATTORNEYS,

Oct. 14, 1947. F. H. JOHNSON ETAL 2,429,186

APPARATUS -F-OR WELDING Filed April 3, 1945 3 Sheets-Sheet 2 j 1 L j fr INV/17015. Fred /7. .701 ysai?,

E. E BY www MM2/11% Mw/naal, 'ly/j K f A' oRNEYa v Oct. 14, 1947. F. H. JOHNSON ErAL.

APPARATUS FoR WELDING .1 l s www n 7M m .m ooo mn n .v 1I WA n m z rif.. e a m 3,. ,Y/ l B. .nV n. of d l l Il... m VM F Patented Oct. 14, 1947 APPARATUS FOR WELDING Fred H. Johnson, Pleasant Ridge, and Chester F.

Leathers, Detroit, Mich.; said Leathers assignor to Progressive Welder Company, Detroit, Mich., a corporation of Michigan Application April 3, 1943, Serial No. 481,676

(Cl. 21B-4) 11 Claims. l

The present invention relates to an electrical control system and in particular provides an improved method of and apparatus for electric welding. In certain of its aspects, the present invention represents improvements over the method and apparatus disclosed and claimed in Leathers and Benkert application, Serial No. 447,006, led June 15, 1942. k

The above-mentioned copending application discloses and claims a welding system in which a complete welding cycle includes a succession of heating and cooling stages, each heating stage being terminated when the work attains a desired temperature, and each cooling stage being terminated when the temperature of the work falls to a desired value. In at least one of the stages, the Work is brought to a welding temperature, in at least one succeeding stage the work is brought to a temperature within the austenitic temperature range and, preferably, in at least one succeeding stage, the work is brought to a temperature appropriate to an annealing action. In the preferred practice of the above system, each heating stage comprises a succession of heating and cooling periods.

The above system is well adapted to the welding of relatively heavy gauge metals, as well as other applications,v and in such cases, the welding current values may be so high that the frequent initiations and interruptions thereof, which take place during the course of a Welding cycle, produce undesirable surges in the power system. As disclosed in the copending application, the welding current is caused to automatically increase, during the initial heating period of the initial heating stage, from a minimum value to a maximum value, whichl action affords a benecial preheating eiect. In accordance with the present invention, it is proposed to eliminate in large part at least the aforesaid undesirable line surges by causing each heating period of each heating stage to begin at a minimum value, of current and to increase to and terminate at a maximum value. It is further proposed to cause each cooling period of each' heating stage to begin at a maximum value of currentand gradually decrease to and terminate at a minimum value of current. With this arrangement, the Welding current varies gradually and relatively smoothly between maximum and minimum values, and abrupt changes in current demand are eliminated. It is further proposed to arrange the system so that current is not interrupted'at the conclusion of each stage, unless or until the current has fallen to the minimum value.

The use of temperature responsive elements in the aforesaid system not only makes it possible to insure the attainment of a proper succession of temperatures in the successive stages of a cycle, but also serves to compensate for Vaziations in the impedance of the Welding circuit encountered in making successive welds. Such impedance changes are produced, as will be understood, by varying physical characteristics of the workpieces, by variations in the degree to which the Workpieces are projected into the throat of the Welding machine, and by other known factors. The impedance changes serve, of course, to vary the value of the Welding current and, consequently, serve to vary the length of time that is required to briner a particular weld zone to a desired temperature. In accordance with the present invention, it is proposed to compensate for such changes in impedance of the welding circuit, so as to reduce the variation in the length of each welding stage. Such action renders more uniform the rate at which' successive welds can be made, as will be understood.

With the above as well as other considerations in view, the principal objects of the present invention are to provide an improved method of and apparatus for electric welding, particularly of the resistance type, which is economical, efficient and reliable; to provide such a system wherein, during the course of successive heating and cooling periods, the current is caused to progressively and smoothly vary between desired maximum and minimum values; to provide such' a system Wherein the current is delivered to the welding circuit through electric discharge apparatus and wherein the variation in the value of the welding current is obtained by phase shifting elements which progressively advance and retire the points in successive half cycles of an alternating current source, at which the discharge apparatus is rendered conductive; to provide such a system wherein means are provided which respond to and compensate for variations in the impedance, or other characteristics, of the welding circuit; to provide such a system wherein the just-mentioned compensation is obtained by providing means which responds to an electrical characteristic of the welding circuit, more particularly the welding current, and which cooperate with the aforesaid phase shifting elements in determining the points in successive half cycles at which the electric discharge apparatus is rendered conductive.

With the above as well as other objects in view, which appear in the following description and in the appended claims, a preferred but illustrative embodiment of the invention is shown in the accompanying drawings, throughout the several views of which corresponding reference characters are used to designate corresponding parts and in which:

Figures l, 2 and 3, collectively, constitute a diagrammatic illustration of a welding system embodying the invention;

Fig. 4 is a view showing the physical relation between the operating coils and the contacts f the series of electromagnetically operated relays employed in Figs. l, 2 and 3. In the latter figures, these coils and relays are shown in separated relation, in order to simplify the drawings; and

Fig. 5 is a view showing the location of a pickup coil relative to the welding circuit.

It will be appreciated from a complete understanding of the present invention that the improvement thereof may be embodied in widely differing welding systems, particularly adapted for widely varying purposes. It is now preferred to embody these improvements in a multistage welding system of the type described in the aforesaid copending application. In an illustrative but not in a limiting sense, the invention is so disclosed herein.

Before proceeding to a description of the drawings, it is noted that the conductors which terminate in the arrowheads at the bottom of Figure l and which are designated, respectively, a, b and c, are directly connected to the conductors which terminate in the correspondingly designated arrowheads at the top of Figure 2. Similarly, the conductors which terminate in the arrowheads designated d, c, f, g, h, z', y and k at the bottom of Figure 2 are directly connected to the conductors having the correspondingly designated arrowheads at the top of Figure 3.

Referring to the drawings, a usual welding transformer WT is provided with a secondary winding, the terminals whereof are directly connected to the electrodes IB and I2 of the associated welding machine. The primary winding of the welding transformer is connected across the line conductors LI and L2, in series with a pair of reversely connected arc discharge devices RI and R2, which may be and preferably are of the type sold commercially under the trade name ignitrons The characteristics of these devices are well known and it is thought they require no description except to note that they are normally non-conductive but that, if an igniting potential is applied between the igniters i and the cathodes c thereof, during a half cycle of the source in which the anodes a thereof are positive, they become conductive and remain so for the balance of the corresponding half cycle of current flow.

In the present system, the points in successive half cycles, of each heating and cooling period of each heating stage, at which the corresponding rectiiiers R1 and R2 are rendered conductive, are determined by a pair of heat control valves V5 and V5, associated with the main rectiners RI and R2, respectively. Valves V5 and Ve may be and preferably are of a usual three-element, gasiilled type. They are provided with potentiometer type grid control circuits comprising a cornmon supply transformer TG and individual transformers T52 and T13. The transformer TIB has a usual centerntapped secondary winding connected to one terminal of the primary windings of transformers T12 and T13. The terminals of the secondary winding of transformer TIB are connected together by a bridge comprising a condenser C10, a resistor r1 and additional resistance represented by a pair of reversely connected, high vacuum tubes Vl and V8, which may be of a conventional type. The remaining terminals of the primary windings of transformers T12 and T13 are connected to this bridge at the point I4, which is between resistor rl and the plate circuits of valves Vl and VS. It will be noticed, accordingly, that by varying the effective values of resistance represented by valves Vl and V3, the relation between the reactance and resistance in the above-mentioned bridge circuit is varied and, consequently, the phase relation between the voltage of transformer 'II 5 and the voltages of transformers T52 and T13 is correspondingly varied, it being noted that transformers T! 2 and T13 are connected to the bridge circuit in opposed relation to each other. The characteristics of the bridge and associated circuits are also such that when the resistance of valves V'I and V8 is at a maximum value, the voltages of transformers T12 and T13 reach values suiiiciently high to fire their corresponding heat control valves V5 and V6 at a point late in half cycles of the welding current. Similarly, when the resistance of valves Vl and V8 is at a minimum, the voltages of transformers r)T12 and T3 reach values sufficient to fire valves V5 and V5 at correspondingly earlier points in half cycles. As will be understood, the firing points of valves V5 and V5 and, consequently, of rectiers RI and R2 may thus be varied over relatively wide ranges corresponding, for example, from a full heat value to a value down as low as 15% to 20% of full heat.

The grid-cathode circuit of valve V1 includes in series relation a resistor T8, a resistor T12, across which a phase shift compensating potential is impressed, an impulsing condenser C13, the potential whereof is successively decreased and increased, and a resistor 19. In addition, a smoothing condenser CII is preferably directly connected between the grid and cathode of valve Vl. The grid and cathode circuit of valve V8 is similarly connected directly across the compensating resistor T12 and the impulsing condenser C13, in series with resistors r11] and r11. Also, a smoothing condenser C12 is preferably connected directly across the grid-cathode circuit of valve V8.

The potentials of condenser C13 and resistor r12 are additive with respect to each other and, consequently, it will be appreciated that, the gridcathode potentials of valves V'I and V8 are at all times equal to the sum of these two potentials. As described below, the potential of condenser C13 predominates and, consequently, when this potential is at a maximum, Valves Vl and V8 are of maximum resistance (affording a minimum heat setting for the system) and when this potential is a a minimum, the resistance of valves V'I and V8 is at a minimum (affording a maximum heat setting for the system).

In the present arrangement, the fall and rise of the potential of condenser C13 is controlled by successively commutating its connections to a discharging circuit comprising potentiometer 114 and regulating resistor TIS, and a charging circuit comprising potentiometer T15 and a regulating resistor rII. The commutation of these circuits is controlled by contacts CRI Ia and CRI Ib of trigger relay CRI I, it being noted that when contact CRlIa is closed, condenser CIS is enabled to charge through a circuit which extends from its positive terminal through conductors 2D, 22 and 24, potentiometer T15, resistor rl'l, conductor 26, contact Cltlla, a 'normally closed transfer switch 28, and .conductor 30 to its negative terminal. On the Other hand, when relay CRII is energized, its contact CRIIa is open and yits contact `CRIIIJ completes a discharging circuit for condenser CI3, which includes conductors 20, 22 and-24, potentiometer M4, resistor r1.6, contact lCRUZ), conductor 32, switch 28 and conductor 30. It will be noticed, also, that the setting of potentiometer rl5 determines the maximum voltage to which condenser CI3 is charged, and that resistor 1I4 determines the minimum voltage to which condenser CI3 is discharged, both of which values are, of course, obviously adjustable, Resistor rIB, in series with potentiometer TM. serves to variably determine the rate at which condenser CI3 is discharged from its maximum to its minimum value, and a corresponding control of the charging rate is provided by resistor rII,

Potentiometers rI4 and rI5 are supplied with voltage from a usual rectifying circuit comprising the continuously energized transformer T20 and a, usual full wave rectifier V9. As illustrated, also, this rectifying circuit includes balancing elements comprising the resistor 1'I3 and condenser C I 4.

As described below, relay CRII is energized at the beginning of each heating period of each heating stage, and is de-energized at the beginning of each corresponding cooling period. During each such heating period, the welding current, consequently, rises from a minimum to a maximum value and during each such cooling period, the current falls from the maximum to the minimum value. The rate of change of the current value may be adjusted, as aforesaid, so as to cause the maximum and minimum values, respectively, to be attained at desired points in the corresponding heating and cooling periods. In general, it is preferred to adjust the minimum value to correspond to approximately 20% of full heat, full heat being dened as usual as being attained by ring the main rect'iers at the power factor angle of the system. Depending upon the current requirements of the welding load, the maximum value of the heat setting may fall anywhere between 50% and 100% of full heat although, when utilized in connection with the hereinafter described phase-shift compensating elements, it is preferred to limit the maximum heat, as determined by potentiometer rI4, to between 80% and 90% of full heat, in order to allow a suicient range for the action of the phase shift compensating elements.

It will be appreciated from the foregoing, that a given heat setting or grid-cathode potential of valves V'I rand V8 causes the main rectiiiers to be fired at corresponding predetermined points in successive half cycles and, consequently, causes, for a given value of impedance in the welding circuit, a corresponding value of welding current to ow. If, on the other hand, the impedance of the welding circuit is varied (as might he caused, for example, by moving the work a greater or lesser distance into the throat of the machine so as to space the welds farther from or nearer to the edge of the work), the saine heat setting is effective to produce a diiferent value of welding current. It is the. function of the potential impressed across the phase-shift compensating resistor rl2 to compensate for such changes.

As shown, resistor TS2 is connected, through an adjustable resistor ri Si ,and a glow ClSChaIge Valve 39, .to lreceive the direct current output of va usual full-ware rectifier comprising transformer TIB and rectier `V;I 0. The primary winding of transformer TIB iis connected to a potentiometer .H8 which, in turn, receives the output of .a Dick-up winding V34 (Figs. 2 and 5). Winding 34 is located in the throat of the welding machine, immediately yadjacent one lof .the bus bars, which extends between one of .the electrodes `and the corresponding secondary terminal of welding transformer WT. With this relation, it will be appreciated that the E. M. F. of winding 34 is directly determined by and is proportional to the current .flowing in the welding circuit. Moreover, by adjusting potentiometer N8, the rectified potential `applied to valve 352 may -be varied with respect to `the potential developed by the pick-up winding 34, `and valve 39 may, consequently, be caused to break down at a value .of welding current corresponding, for a given value of welding current impedance, to a desired percentage of full heat. Further, by adjusting potentiometer rIS., a desired percentage of the rectiied voltage may be impressed across resistor M2. It is now preferred to use an adjustment which causes valve 39 to break down at a value of welding current corresponding to a maximum impedance in the welding circuit and to the maximum heat setting, as determined by potentiometer M4, .and which further causes the resultant potential impressed across resistor rI2 to be approximately 10% of the potential of condenser CI3.

It is thought that the remaining details of the operation of the automatic heat changing elements and the compensating elements, as well as the other details, which have to do with the mechanical operation of the machine, the control of the length of the cycle as a whole, the control `of the several heating and cooling stages, and the control of the lengths of the heating and cooling periods of each stage may best be understood from a ydescription of the operation of the system as a whole. It is noted that the system as a Whole, except in the respects which afford the phase shift compensation and the progressive change in heat setting during successive heating and cooling periods is substantially the same, unless noted otherwise, as the system disclosed in the aforesaid copending Leathers and Benkert application.

Assuming it is desired to eiTect a welding operation, the system may be conditioned for operation by closing the illustrative line switches LSI .and LS2, which connects line conductors LI and L2, in each of Figs. 1, 2 and 3, to a usual alternating current source of supply. Upon being energized, line conductors LI and L2 complete obvious energizing circuits in Fig. l., for the primary windings of transformers T3, T6 and TI I, associated, respectively, with Valves VI, V2 and V4. These valves, as well as valve V4, may be and preferably are of the usual threeelement, high vacuum type and, as will be understood, Vthey may be provided with usual heating circuits which bring the cathodes c thereof to emissive temperatures. These heating connections have been omitted` from the drawings to simplify them. Upon being energized, transformers T3, T5 `and TII apply blocking potentials to the grids of the associated valves, and. also charge up the associated grid condensers CI, C3 and C5. These actions serve to render the corresponding valves non-conductive, as will be understood.

In Fig. 2, the energization of line conductors LI and L2 supplies energy to transformer T2I which, through a usual full wave rectifier, applies a D. C. potential between terminals 50 and 52, thereby affording a source f direct current for operating the windings 54 and 56, associated with the ratchet unit 58.

In Fig. 3, the energization of line conductors LI and L2 supplies, through obvious circuits, energizing current for the lament transformers TI4, TI5 and TI3, associated. with the filament circuits of valves V5, V6 and VID. An obvious circuit is also completed for transformer TI 1, which supplies filament current for valves V1, V8 and V9.

The energization of line conductors LI and L2 (Fig. 3) also energizes transformer T20, which thereupon, through rectifier V9, applies a fixed potential across potentiometers 1I4 and rl5, Which action results in charging condenser CI3 to its maximum value, the charging circuit extending through resistor 'ri5 and the now closed back contact CRI Ia of relay CRI I. Under the conditions stated, no potential is applied across resistor 1I2 and, consequently, the grid-cathode potentials of Valves Vl and V8 are directly determined by the potential of condenser CI3. Being at a maximum value, substantially a maximum resistance is afforded by valves Vl' and V8.

The energization of line conductors LI and L2 (Fig. 3) also energizes transformer TIS, which, through the previously described bridge circuit, energizes grid transformers TI2 and TI3 associated with Valves V5 and V5. These potentials, which are at a phase position corresponding to approximately a minimum heat setting of the system, tend, during successive half cycles, to re valves V5 and V6. Under the conditions stated, however, the plate circuits of these valves are interrupted at the now open contacts CRIZa and CRi2b of relay CRZ, so that n0 current is passed by these valves.

Assuming it is desired to effect a welding operation, the worlrpieces W may be introduced between the electrodes i0 and. I2, after which the pilot switch PI may be closed. Closure of this switch may in any desired manner serve to cause the electrodes to move into clamping engagement with the work. In the illustrated system, it is assumed that the welding machine embodies the combined air and hydraulic electrode control mechanism described and claimed in the copending application of Hugo Purat, Serial No. 466,981, flied November 23, 1942, now Patent No. 2,413,783, dated Jan. '7, 1947, which mechanism includes air valves Si! and G2 and a hydraulic valve 64, the sequencing of which is briefly described herein and is more completely described in the copending application. Briey, closure of pilot switch PI directly energizes air valve 60 and, through the now closed back contacts CRSD and CRZZJ, also energizes hydraulic valve 64. Further, through the now closed contact CR3b and conductor t5, such action energizes the other air valve 52. As is described in more detail in the Purat application, the energization 0f these three valves applies pressure to the electrodes and causes them to move into clamping engagement with the work.

Closure of pilot switch PI, also, through conductor 68, directly energizes transformers TI and T2, associated with valve VI. Upon being energized, transformer T2 opposes the previously energized blocking transformer T3 and enables the energy stored in condenser CI to discharge through the associated resistor rI At the expiration of a discharge period, proportioned to be long enough to enable the full movement of the electrodes into the clamping position, valve VI becomes conductive, enabling the now energized plate transformer TI to pass Current therethrough and energize control relay CR2, Upon being energized, relay CR2 closes its contacts CRZa and CR2c and opens its back contact CR2b. Closure of contact CRSa prepares a circuit through conductor '10, which extends through to Figure 2, for control relay CRT, which action is without immediate effect, and also makes a holding circuit for air valve 62. The opening of contact CRb de-energizes the hydraulic valve 64, which prepares the system for the application of the normal or welding pressure, but is otherwise without effect, in so far as the operations pertinent to the present invention are concerned. So long, accordingly, as pilot switch PI is held closed, the electrodes remain in engagement with the Work with a light or preliminary pressure.

The above-mentioned closure of contact CR2c completes energizing circuits for transformers T4 and T5 associated with the anode and grid circuits of valve V2. Upon being energized, transformer T5 opposes transformer T6 and enables the energy stored in condenser C3 to discharge through the local circuit including resistor r2. After a corresponding discharge period, the grid of valve V2 attains a potential at which this valve is conductive, enabling transformer T4 to pass current therethrough and energize control relay CRS. Upon being energized, relay CRS closes its contacts CR3a and 0R30 and opens its back contact CRSD. The latter action is without effect, since valve 52 is maintained energized through the now closed contacts CRZa and CR3a and valve S@ is maintained energized through the still closed switch P I.

Closure of contact C12-3c prepares a circuit for the welding pressure valve 8D and for transformer Tl, which circuits remain incomplete, however, pending initiation of the actual welding cycle. The purpose of the contact CRSC is to prevent application of the weld pressure unless and until the means which responds to valves 60, 52 and E4 has had time to bring the electrodes into engagement with the work, and the timing of condenser C3 is, of course, adjusted to insure this relation.

Assuming it is desired to initiate the weld, any one of the three pilot switches P2, P3 or P4 may be closed, depending upon the number of stages which it is desired to include in the full welding cycle, In the illustrated system, switch P4 is effective to produce a cycle having three stages, for example, a welding stage, a grain refining stage (carried into the austenitic temperature range) and an annealing stage. Closure of switch P2 is effective t0 produce the same cycle except that the annealing stage is omitted, and switch P3 is effective to produce a cycle in which both the annealing and grain refining stages are omitted, leaving only a welding stage.

Assuming switch Pd is closed, to produce the full three stage cycle, such action directly completes a circuit for relay CRI, subject only to the now closed baclr contact CR'a of relay CRT. Upon being energized, relay CRI closes its contacts CRla, Cl-'tlb and CRIc. Contact CRib directly completes a holding circuit for relay CRI, in parallel with switch P4, which may thereupon be released to the open position without affecting the cycle now in progress; and in conjunction with contact CR la, completes a holding circuit for air valve 6U.

The closure of switch P4 also completes a kcir-- cuit (which is held closed after the opening of switch Pfi by contact C'Rlb) for the weld pressure valve 23B and transformer T-I associated with valve V3. Upon beingenergized and in the manner described in the aforesaid Purat application, weld pressure valve A8i! increases the pressure on the electrodes to a value suitable for welding purposes, thus placing the system in condition for the actual initiation of the iioW of welding current.

Upon being energized, transformer T'I applies anode potential to the normally conductive valve V3 and passes current therethrough to energize control relay CR4. Upon being energized, relay CR4 closes its contacts CRM and CRM), Which contacts are connected in series relation with each other. Closure Aof contact CRa completes a circuit through conductor '82 for the coil of relay CR'IZ, which circuit also includes, in series relation, the now closed back contacts CRBa and CRIa of relay CRS and CRT. Closure of contact CRa in conjunction with contact CRIIb energizes the previously identied phase shifting relay CR! i, through a circuit which includes, in series relation, the now closed back contacts CR5a and CRla of relay CRE and CR?.

Upon being energized, relay CRI2 closes vits contacts CRIZa, CRIZb and CRIZc. Contact CRIZa completes a self-holding circuit for relay CRM, which holding circuit, however, includes the now closed Contact CRIb of relay CRI. Closure of contacts CRI2b and CRI2c (top of Fig. 3) applies anode potential to the ring valves V5 and V5. The circuit for valve V5 extends from line conductor LI through conductor 9i! (bottom of Fig. 2 and top of Fig. 3), valve V5, contact CREb, conductor 82 (bottom of Fig. 2 and top ci' Fig. 3), ianiter z' and cathode c of main rectifier Rl, and thence through conductor 84 and 'the primary of the Welding `transformer WI to line conductor L2. Closure of contact CRI 2c completes a similar circuit through which potential is applied across valve V6 and it will loe'noted that in these circuits, valves V5 and V6 are connected, respectively, in parallel with the anodecathode circuits of main rectiers RI and R2.

It will be recalled from previous description that, during each successive positive half cycle and, with the initial adjustment of the system, at a point therein which corresponds to about 20% full heat, transformer TI2 applies a conductive potential to the grid of valve V5, tending to render this valve conductive. At a corresponding point in each negative half cycle, a conductive potential is applied to the grid of valve V5 by transformer T23. Depending upon Whether the closure of contacts CRI2b and CRIZc takes place during a positive or a negative half cycle, therefore, either' main rectifier RI or main rectier R2 the rst main rectifier to be fired. Assuming such closure occurred during a positive half cycle, the potential applied to valve V5 passes current therethrough and passes an igniting current between the igniter i and the cathode c of rectiiier RI, which action, as will be understood, fires this valve and initiates the ilcw of Welding current. At the corresponding point in the succeeding negative half cycle, rectier R2 is ired through valve V6 and so long, therefore, as contact CRIZb and CRlZc remain closed, rectifiers RI and R2 are effective' to pass current to the welding circuit 10 at a heat setting determined by the phase shift adjustment of transformers TI2 and TI3.

Upon being energized, as aforesaid, and simultaneous-ly with the energization of relay CRI2, relay CR'II opens its backA contact CRIIa 'and closes its front contact CRI Ib. The former action interrupts the circuit for condenser CI3, which includes the minimum heat `setting potentiometer rl 5. Onthe'otlier hand, closure of contact CRI Ib completes the previously described discharge circuit for condenser CI3, which circuit includes potentiorneters rI4 and TIS and through which circuit condenser C13 is enabled to gradually discharge to the `minimum potential determined by potentiometer TIA., `at a rate determined by resister rit. iin response to this action, the potential across condenser CI3 gradually falls from its norma-l or maximum value towards its minimum .faluc. '.lhe progressive decrease in potential across condenser CI-3 is reflected as a decrease in the negative grid bias potential of valves V1 and V8, which action correspondingly and gradually decreases the resistance of these valves and alters the ratio of reactance to resistance in the bridge circuit to which transformers r'II2 and TIB are connected. As aforesaid, this shift gradually advances the points in successive positive and nega-tive .half cycles atv `which valves V5 and V6 are rendered conductive. This, in turn,advances the pointin such yhalf cycles at which rectiers RI andRZ are red, and correspondingly increases the value of the .welding current.

In addition .to the operations described above, closure Vof contacts CRIIa and CR4b of relay CR4 completes .an .energizing circuit for the grid-trans` former T8 associated with valve V3, which action blocks this valve and interrupts the supply of current to relayfCR from anode transformer T1. In `response Lto 'this action, fthe kenergy stored in they coil of relay CR4 startsto discharge through the associated condenser C4 and at the expirationofa predetermined heating period, relayCRl resumes 'the ie-energized position, opening its contacts 'CRIIa and CRM). f In viewfof the holding circuit 'afforded by contactCRI2a, .this action is without effect on the starting relay CRI'Z, which, consequently, continues tovmaintai-n anode potentialscn firing rvalves `V5 and V6. The opening of contacts CRlIa andCRIlb does, however, de-energize 4relay CRII, which thereupon reeopens its contact CRIIb yand Areclose's its contact CRIIa. The former action interrupts the previously described discharge circuit vfor condenser-CI3, vReclosure of 'contactfCRI la recornpletes the original charging circuit for condenser 1CI3, lthrough potentiometer rI5, initiating a gradual charging of vcondenser CIS and a consequent gradual increase in the grid-cathode potentials of Valves V1 and V8. This action progressively increases the'resistance of these valves and, consequently, progressively retires the ring points of the main rectifiers RI and R2.

The opening of contacts CRM and CRM) also de-'energizes grid transformer'TB, enabling the ener'gynow stored in condenser C5 to Start discharging through the local circuit, including resistor r3. At the expiration of a predetermined discharge period, valve V3 again becomes conductive, enabling transformer Tl to 1re-energize relay CRL This action duplicates the action which resulted from the initial energization of relay'CR4 in so far as concerns the progressive advance of thelieat setting of valves V5 and V6, but does not affectrelay CRI 2, since this relay is already energized. The re-energzation of relay CR4 also re-energizes transformer T8 and applies a blocking potential to the grid of Valve V3. This action again initiates a timing out of relay CRO, at the expiration of which it resumes the de-energized position and initiates a retirement of the heat setting of valves V5 and V6. So long, therefore, as transformer Tl remains energized, relay CRA pulsates between an energized position and a de-energized position, the energized interval being determined by condenser C4 and the deenergized interval being determined by condenser C5. Each energized interval, in turn, aiords a heating period during which the heat setting of valves V5 and V5 is increased from a minimum to a desired maximum and each de-energized interval affords a cooling period during which the heat setting of these Valves is retired from the desired maximum to the minimum. As will be understood, depending upon the relative durations of the energized and de-energized intervals of relay CRl, and the charging and discharging times of condenser Cl3, all or any desired part of each heating period may be utilized to bring the welding current from the minimum to the maximum and all or any desired part of each cooling period may be utilized to bring the welding current from the maximum to the minimum. In most cases, an adjustment is used in which current ows at the maximum value for a substantial part of each heating period and flows at a minimum Value for a substantial part of each cooling interval, it being understood that the term cooling period is herein used in accordance with trade usage as referring to the relation between the electrodes and the work rather than to temperature conditions at the junction between the workpieces.

As thus far described, accordingly, closure of the pilot switches PI and P4 has brought the electrodes into engagement with the work with a welding pressure, and has initiated a ilow of welding current to the work, which flow of welding current pulsates between minimum and maximum values, which action continues until interrupted by the temperature responsive elements described below, which, in conjunction with the ratcheting device 58, serve to determine the number of stages, temperatures attained in the successive stages and the temperatures attained in the intervals between successive stages.

The temperature responsive apparatus may be and preferably is arranged, as described in detail in the aforesaid Leathers and Benkert application and, in order to simplify the drawings and description, this apparatus is diagrammatically shown in Fig. 2 as a pyrometer element |00, having an input terminal |02, which is directly connected to the electrode |2 and having a series of four input terminals |04, |06, |08 and ||0. Terminals |04, |06 and |08 are connected, respectively, to terminals bl, b3 and b5 of a commutator operated by the above-mentioned ratchet unit 58. The remaining terminal ||0 is connected to commutator terminals b2, b4 and 136. The arm ||2 of the commutator unit is connected to a thermocouple element 4 such, for example, as constantan, which is embedded in the electrode i2 in the manner described in the Leathers and Benkert application. The constantan and the electrode |2 cooperate to form a thermocouple which responds to the temperature attained at the junction between the work and the electrode l2, which relationship is generically referred to herein as the temperature of the work. It will be understood that the pyrometer |00 is provided with adjusting elements, not shown herein, which cause it to correspond to a succession of diiferent temperatures, depending upon which one of the commutated input terminals |04, |06, |08 or |0 the common terminal |02 is connected to. When terminal |02 is connected to terminal |04, these internal adjustments cause the pyrometer to respond to a temperature corresponding to a welding temperature at the junction between the workpieces. When terminal |02 is connected to terminal |06, the pyrometer responds to a temperature falling in the austenitic temperature range, appropriate to a grain refining heat of the work. When the terminal |02 is connected to terminal |08, the pyrometer responds to a temperature appropriate to an annealing operation, which temperature, for example, may be in the immediate region of or somewhat below the socalled critical temperature of the work. When the terminal |02 is connected to terminal 0, the pyrometer responds to a relatively low temperature well below the annealing temperature of the work and this is the temperature to which the work is allowed t0 fall in the intervals between successive stages.

Each time one of the aforesaid adjusted temperatures is attained, causing the pyrometer to respond, it closes its sole contact, designated PY@ near the bottom of Fig. l, which action, as described below, terminates the heating stage then in progress. Each time the contact PYa opens, a corresponding cooling stage is terminated.

The ratchet element 58 may and preferably does correspond to the similar element described in the Leathers and Benkert application and comprises a toothed drum which is continuously urged in a clockwise direction by a coil spring. The drum is provided with an advancing pawl i20, controlled by a solenoid 54 and is provided with a resetting pawl |22, controlled by a solenoid 56. In addition, the drum has associated with it a limit switch |24, which is disposed to be opened by it when and so long as the drum occupies the normal or starting position, in which the associated commutator arms |29 and ||2 occupy the illustrated positions out of engagement with the terminals disposed to be engaged thereby. The terminals al, a2, etc., associated with arm |25, are associated with the high pressure or forging relay CRB, the action of which is described below.

Continuing now with the description of the operation, the initial energization of relay CRl, which results from the closure of pilot switch P4 at the beginnin I of the welding cycle, also causes this relay to close its iront contact C'Ric. This action completes a circuit through the new osed series connected back contacts ORSI? and CRSb for the winding 54, associated with the advancing pawl of the ratchet unit. This circuit serves to apply the rectified potential between terminals 50 and 52 to winding 54, through conductors |30, |32, a portion of line conductor Li and the above-mentioned contacts. energized, winding 54 advances pawl moves the ratchet unit to its rst operaV tion, in which arms ||2 and |26 engage nals bl and a|, respectively. The latter enga 1 ment is without effect, but the former engagement connects input terminals |02 and iGfi to the thermocouple aiorded by the electrode i2 and the embedded element ||4. The conti "ing fic-w of welding current initiated, as descr causes the temperature of the work to progressively rise, which rise in temperature, taking Upon being 13 place during the course of successive pulsations of the current between the aforesaid maximum and minimum values, is reflected as an increasing thermoelectric M. F. between terminais I92 and IM. When this temperature reaches Welding Value, appropriate to the formation of a Weld nugget between the Worlrpieces, pyrometer Id responds and immediately cio s its contact PYa. Referring to Fig, l, this action immediately energizes relay CRiS and transformers T9 and Tt), associated with valve V4. Upon being energized, relay CR@ opens its contacts and CReb and closes its contact CRiic. opening of contact interrupts the originally traced energizing circuit for relay CRiZ, which relay does not immediately drop out, however, because of the holding action of condenser connected in parallel with the coil thereof. The opening of contact Clif?) interrupts the previously traced energizing circuit for solenoid ld, enabling the spring associated with pawl to withdraw .it to a position in which it engages behind the next successive tooth of the ratchet drum, this action being preparatory to an advancing movement of the ratchet unit.

Upon being energized, transformer Tl opposes the continuously energized transformer TII and enables the lenergy stored in condenser C6 to discharge through the local circuit comprisingresistor rfi. At the expiration of a short interval, of the order of a cycle or two, sufficient only to allow the abovementioned withdrawal of the pawl valve V4 becomes conductive. This action enables transformer to pass current therethrough and energize relay CR, which thereupon opens its contacts CRlia and Cilc and closes its contact CRc. The opening of contact CRE?) is. Without immediate effect, since contact CRSb is now open, but the closure of contact CRE@ completes a circuit, through con# tacts CRIc and CREia, forv winding 54, which thereupon actuates pawl and advances the ratchet unit to its second position in which arm I I2 engages terminal b2 and arm 26 engages terminal al The latter action completes a circuit for relay Cawhich thereupon closes its sole contact CH3-ff, (near the top of Fig. 2). This action energizes the forging pressure valve 52, resulting, as described in the aforesaid Purat application, in the emplicationA to the work of a substantially higher or forging pressure.

The transfer movement of arm II'2 interrupts the input circuit between terminals S62 and its and completes an input circuit between terminals I B2 and IIl. This transfer movement is quite rapid and, due to the sluggish character of the pyrometer IDS, is completed before contact PY-t has an opportunity to open. As soon as the transfer movement is completed, the pyrometer is readjusted to a materially lower temperature, as aforesaid, and consequently maintains its contact PYa closed until such materially lower temperature is attained.

The opening of contact CRSa immediately interrupts the energizing circuits for transformer TI and for phase shifting relay CRI I. It Will be noted that this action may take place either during a heating period or during a cooling period, since it is found in practice that the temperature of the Work may continue to build up at least throughout the initial portion of a cooling period. Consequently, the critical operating temperature, at which the pyrometer responds, may be attained during a cooling period. If the pyrometer responds, during a. cooling period, the opening of contact CRSa is without immediate effect, since under these conditions, relays CR4 and CRII are already dre-energized and the phase shifting elements comprising condenser CI3 are already proceeding to reduce the value of welding current towards its minimum value. In such instance, the opening of Contact CRBa serves only to pre- Vent the re-energization of relays CR4 and CRI I and, consequently, prevents the initiation of a heating period at the conclusion of the cooling period then in progress.

On the other hand, if the pyrometer responds yduring a heating period, the consequent opening of contact CREG, immediately de-ene'rgizes relay CRI I and initiates a retirement of the phase shift position. This action takes place regardless of Whether the current has yet attained its maximum Value or is of only an intermediate value.V In such case, the opening of contact CREa initiates a gradual reduction in the current and also, as before, prevents the initiation of a succeeding heating period. yIn such case, also, the de-energization of transformer TI is without immediate effect on relay CR4 ybecause of the holding action of condenser C4, but does prevent the re-energization of relay CRII.

The minimum value of welding current is well below the value needed to maintain the work at the welding temperature, and, consequently, the operation of the pyrometer, as aforesaid, initiates a cooling off of the Work. To hasten this cooling action, it is preferred to completely interrupt the flow of Welding current and for this purpose, as aforesaid, the operation of the pyrometer serves to interrupt the initial energizing circuit for the starting relay CRI2. The condenser I5|J associated with this relay, however, maintains it in the energized position long enough to allow the phase shifting elements to reduce the current to its minimum Value. Condenser |50 is, consequently, preferably adjusted to give a holding interval yslightly longer than the time required to allow phase shifting condenser CI3 to discharge from its maximum potential to its minimum potential. When the temperature of the work has fallen to the aforesaid low value, well below the annealing temperature of the work, the pyrometer contact PYa resumes its open position, which action immediately de-energizes relay CRB, transformer T9 and transformer TIO. The de-energization of relay CRB causes its contacts CRSa, CRIb and CRSc to resume the illustrated positions. Contact CR'Ga, upon reclosure, again prepares a, circuit for the starting relay CRI 2, which circuit, however, remains open at contact CRlIa. Reclosure of contact CRSb prepares the original energizing circuit for ratchet Winding 54, and the opening of contact CRSa, interrupts the last traced circuit for winding 54. In response to this action, pawl |20 is withdrawn to a position in which it engages behind the next ratchet tooth, preparatory to the next ratcheting step.

The de-energization of transformer TI!! enables the continuously energized transformer TI I to apply a blocking potential to the grid of valve V4, which action interrupts thel original energizing circuit for relay CRS and enables the energy stored in the coil thereof to discharge through the local circuit comprising condenser C6. At the expiration of ra short period, suficient only to allow the above resettingmovement of the ratchet pawl |20, relay CRB resumes the de-energized position, restoring its contacts CREa, CRSZ? and CR5c to the illustrated positions. The transfer action of contacts CR5b and CR5c recompletes the original energizing circuit for Winding 54, which thereupon acts to advance the ratchet drum to the third operating position in which arms ||2 and |26 engage terminals b3 and a3, respectively.

When arm |26 moves away from terminal a2, it interrupts the circuit for relay CRB, which thereupon resumes the de-energized position. This action de-energizes the forging pressure Valve |52, restoring the electrode pressures to the lower or welding value. In the sequence now being described, the engagement of terminal a3 by arm is without effect.

The engagement of arm ||2 with terminal h3 connects terminals |02 and |05 to the thermocouple, which terminals, as aforesaid, adjust the pyrometer to a temperature appropriate to a grain refining heat. This adjustment makes no change in the now open position of the pyrometer contact PYa, since this grain refining temperature is well above the pre-existing temperature of the Work.

The reclosure of contact CR5a completes the original energizing circuit for transformer TT, associated with valve V3. Upon completion of this circuit, relay CRll is again energized and initiates the second heating stage in the manner described with reference to the first stage. It is believed that it will be evident that during this second stage, relay CR|| is again pulsated between energized and de-energized conditions, so as to provide successive heating and cooling periods in the same manner that these periods are provided during the initial or welding stage. Also, when the work attains the grain refining temperature, the pyrometer responds, as before, to initiate the second cooling stage. In this case, the initiation of the second cooling stage involves advancing the ratchet arms into engagement with terminals b4 and a4, respectively. The latter action again energizes the forging pressure relay CRS, which acts to apply the forging pressure to the work. The engagement of terminal b4 again adjusts the pyrometer for its minimum temperature corresponding to the temperature attained at the expiration of the first cooling stage.

At the expiration of the second cooling stage,

also, the third heating stage is initiated in the same manner that the second stage was initiated, except that in this case, the ratchet arms ad- Vance into engagement with terminals b5 and a5, respectively, it being understood that terminal b5 adjusts the pyrometer for the third or annealing stage.

During the course of the third heating stage, relay CR|| is again pulsated in the previously described manner, it being noted, of course, that the second stage may be expected to include fewer heating and cooling periods than the first stage, and that the third stage may be expected to include fewer such periods than the second stage.

The third cooling stage also proceeds in the previously described manner and at the conclusion thereof the system is reset to its original condition as follows:

When the minimum temperature corresponding to the pyrometer input terminal |0 is reached, the pyrometer Contact PYa again opens and, as before, by virtue of the reclosing of contacts CRS?) and 0R51), the ratcheting unit is advanced to its seventh position, in which the arms |26 and ||2 engage terminals aT and bT. The

16 terminal bT is an inactive terminal in the operation now being described.

The movement of arm |26 out of engagement with terminal a6 again de-energizes the forging pressure relay CRB, restoring the electrode pressure to the normal or welding value. Its movement into engagement with terminal aT completes the circuit, subject to the now closed parallel-connected contacts CR2a and CRSa (top of Fig. 1) for the resetting relay CRT. In response to this action, relay CRT assumes the energized position, closing its contacts CRTb and CRTC and opening its contact CRTa. The opening of contact CRTa immediately interrupts the circuits for relays CRl, CR|2 and CRII, and also interrupts the circuits for anode transformers Tl and TT, associated, respectively, with valves VI and V3. The interruption of the circuits for relays CRI2 and CRI at contact CRTa, prevents the re-energization of these relays, which re-energization normally occurs at the expiration of a cooling stage, and thus terminates the welding cycle. Closure of contact CRTb completes a circuit for the resetting winding 55, associated with the ratchet unit, which thereupon withdraws the pawl |22 from engagement with the ratchet drum, preparatory to the return movement of the drum.

It is to be noted that the de-energization of relay CR5, which resulted from the opening of contact PYa. at the conclusion of the iinal cool period, not only energizes relay CRT, as aforesaid, but, at contact CRSa, tends also to recomplete the circuit for transformer TT, thereby tending, through the consequent sequential energization of relays CR4, CRI and CRIZ, to re-initiate the now of welding current. Such recompletion may be prevented by providing a slight delay in the closing movement of contact CRSa, sufficient to enable the opening of contact CRTa to precede such closure. Even in the absence of such' time delay on contact; CRSa, the recompletion of the circuit for transformer TT is of a momentary character only. In practice it is found that by virtue of its relatively heavier character, the starting contactor CR|2 does not have time to reach the closed position before its circuit is opened by the opening of contact CRTa. If desired, suitable means may be provided to delay the closure of contactor CRI'Z. For example, such means may comprise the condenser which may be properly related to the other constants of the associated circuit so as to afford a desirably short closing time delay and still afford the previously described desired delay in the opening of contacter CR|2 at the conclusion of the respective heat stages.

Continuing with the resetting action, the deenergization of relay CRI causes its contacts CRa, CRIb and CR|c to re-open. The latter contact de-energizes operating winding 54 of the ratchet unit, and enables the associated return Spring to restore it to the illustrated starting position, in which the arms |26 and ||2 are out of engagement with their associated commutator terminals. At the conclusion of this return movement, the cam |23 on the ratchet drum opens the limit switch |24, thereby de-energizing the reset winding 56.

The return sweep of the arm ||2 sequentially re-connects the terminals ||0, |08, |06 and |04, which action is, however, without effect, since the work is now below the temperatures to which these terminals correspond. The return sweep of the arm |26 causes it to momentarily complete circuits for the forging pressure relay CRB, but

this return movement is so rapid that fthe relay does not remain energized long enough to have any effect. The return sweep of the arm |26 also interrupts the initially traced circuit for relay CRT, which action is, however, kwithout efrect in view ofthe holding effect of the now closed contact CRlc, which is connected in parallel with arm |26.

Contacts CRla and CRIb, in opening, further interrupt the circuit for 'transformer TI and also interrupt the previously tracedholding circuits in parallel with the pilot switches PI and P4. In addition, the opening of contact CRlb interrupts the circuit for the welding pressure valve 80, whichthereupon relieves the welding pressure from the work and restores this pressure to the initial low Value.

The opening of contacts CRIa and CRlb also interrupts the remaining energizing vcircuit, for the winding of the Valve 50. This action, as described in the Purat application, is preparatory to the return or separating movements of the electrodes.

The de-energization of transformer Tl deenergizes control relay CRZ, which thereupon resumes the de-energized position, opening its contacts CR2a and CR2c `and reclosing its contact CR2b. The re-opening of contact CR`2a is without effect, but the closure of contact CR2b reoompletes a circuit for the hydraulic valve 64, which action, in conjunction with the cie-energization of valve B0, serves, as described inthe Purat application, to separate the electrodes from the work. The opening of contact CRZc interrupts the circuitsv for transformers T4 and T5, associated with valve V2 and control relay'CR3. These actions cooperate to interrupt the flow of current to relay CR3, whereupon the energy stored in the coil thereof starts to discharge through the associated condenser C2. At the expiration of a relatively short period, suliiciently long to enable the full separating movement of the electrodes to take place, relay CR3 resumes the de-energized position, opening contacts CRBa and CR3c and reclosing contact CR'3b. The opening of contact CR3c and the closure of contact CR3b is withouteffect, since the associated circuits are open at other points. The opening of contact CRSa interrupts the only remaining energizing circuits for valves 62 and 64, thereby restoring the combined air and hydraulic electrode control system to its original condition, all as described in detail in the Purat `applica-tion. The above-described resetting operations, consequently, lserve to restore the vsysternto its nor mal condition, in readiness 'for the next welding operation.

A weldingoperation comprising only two heating and two cooling stages may be effected by closing pilot switch P2. The resulting operations are the same with the exception that such closure also energizes relay CRS (top of Fig, 1). Upon being energized, this relay closes its sole contact CRBa, which serves, as shown in Fig. 2, to connect the coil of relay CRT to the commutator terminal a5. It will 'be recalled that the arm 126 moves into engagement with this terminal immediately following the conclusion `of the second cooling stage. Since, also, the energization of relay CRI terminates the weldingoperation, as described above, it will be appreciated that by energizing this relay, following the second cooling stage, thecycle may be limited, as aforesaid. Also, a single stage operation can be eected by closing pilot switch P3, which action, in addition to the other described oper` ations, energizes relay CRN. If energized, relay CRHJ closes its contact CRIDa and connects the coil of relay CR'I `to the commutator terminal a3. It will be recalled that this terminal is engaged at the conclusion 4of the first cooling stage.

Considering now the operation of the phase shift compensating elements comprising the resistor rl2, it will be appreciated that during the gradual discharge of the impulsing condenser C'l3, which takes place, as aforesaid, throughout at least the rinitial portions of each heating period of each heating stage, the kglow discharge valve 39 remains non-conductive, and consequently the negative grid bias of valves Vl and V8, kis determined entirely by the changing charge on condenser LC13. This changing charge, in turn, provides a corresponding progressive advance of the heat setting of the system and ycauses the flow of a value of welding current which is determined by the impedance of the welding circuit. When a critical value of vwelding current is rattained, corresponding, for example, to the adjusted maximum heat setting of valves V5 and V6, under conditions of maximum impedance of the welding circuit, valve 39 breaks down and applies an adjusted proportion of the potential of the pick-up winding 34 across the resistor r'lZ. This latter potential, as aforesaid., is additive with respect `to :the then existing potential of condenser Cl3 and serves -to correspondingly increase the `negative grid bias of 'valves V1 and V8, thereby correspondingly retiring the heat setting of valves V5 and V6. The value of welding current that hows, consequentlyy is determined by -a heat setting which, in turn, is determined by the` added .potentials of condenser CI 3 and resistorrl 2.

Assuming by way of a second example, that the welding circuitcontains a lower value of impcdan'oe, vvalve 3'9 may be expected to break down before condenser C13 has discharged to a value correspon-ding to the maximum heat setting. In such case/condenser Cl3 continues to discharge after Valve -39 has broken down. During this continued discharge, the Welding current tends to continue to increase by Virtue of the decreasing negative bias eiect of condenser C13. Any tendency, however, for the welding current to increase, tends to increase the output of the picle-up winding 34 and to proportionately iincrease *the potential across resistor N2. Any such r'increase in the potential across resistor rfl would increase the negative bias of valves Vl and V8, which action, in turn, would retire the heat setting. The component of voltage 'introlduced into the heat controlling circuit from the pick-up winding 34 thus functions to hold the actual 'welding current to ya value corresponding to the maximum heat setting determined by condenser Cl3. This .is for the reason, by 'way of a summary, that any tendency of fthe welding current to assume a value above or below the predetermined value, is met by a corresponding decreasefor increase in the potential across resistor M2, which latter change is in adirection to oppose the change in the welding current. It -willfbe appreciated that since the herein provided ycontrol elements may be regarded as inertialess, the corrective eifect appears virtual-ly simultaneously with the Lincrease or decrease in welding current which produces .it and, conselqu'eritly, there yis only a negligible tendency of 'the .system .to `hunt between .welding current Values above o'r below kthe desired'normal value.

At the conclusion of each heating period, the charging up of condenser CIB increases the negative bias on valves V1 and V8, which increase is opposed by the compensating effect of resistor T12. Within the limits of its effectiveness, resistor rl2 opposes the decrease in value of welding current. As aforesaid, however, it is preferred to so adjust the system that under normal conditions the potential across resistor r|2 is of the order of of the maximum heat setting, or minimum potential, of condenser CI3. The gradual charging up of condenser CI3 thus overcomes the compensating eiect of resistor r|2 and, at some intermediate stage of the cooling period, brings the welding current to a value too low for the output of winding 34 to sustain a discharge through Valve 39. At such time, valve 39 again isolates resistor r|2 from the pickup winding and maintains it isolated until the critical point in the next heating stage is attained.

Although only a single embodiment of the invention has been described in detail, it Will be appreciated that various changes may be made therein within the spirit and scope of the invention.

What is claimed is:

l. In a control system for controlling the now of current between a load circuit and a source of alternating current, the current in said load circuit serving to heat Work, the combination of means for causing current flow te said load circuit during each of a succession of on periods each comprising a controlled plurality of half cycles of said source, means for causing the current during each on period to gradually increase from a starting value to a maximum value and to thereafter decrease to a minimum value, means for preventing said current flow during off periods intervening between said "on periods, each oi period comprising a controlled plurality of half cycles of said source, control means operable to initiate the rst said on period, and means operably responsive to the heating or" the work for controlling at least certain of said second and third-mentioned means.

2. In a control system for causing flow of current to a load circuit from a source of alternating current during successive on periods and for preventing such iiow during successive 01T periods, said periods each comprising a plurality of cycles of the source and said flow of current serving to heat work, the combination of electric discharge means actuable to pass current during each half cycle of each said on period, means for variably controlling the portion of each such half cycle throughout which said discharge means passes current to thereby cause the current so passed to gradually increase from a starting value to a maximum value and to thereafter decrease to a minimum value, means for preventing said current flow during said o periods, control means for initiating the first said on period, and means operably responsive to the heating of the work for controlling at least certain of said second and third-mentioned means.

3. In a control system for causing now of current to a load circuit from a source of alternating current during successive on periods and for preventing such ow during successive oi periods, said periods each comprising a plurality of cycles of the source and said iiow of current serving to heat work, the combination of electric discharge means actuable to pass current during each half cycle of each said on period, means for variably controlling the portion of each such half cycle throughout which said discharge means passes current to thereby cause the current so passed to gradually increase from a starting value to a maximum value and to thereafter decrease to a minimum value, means for preventing said current flow during said off periods, control means for initiating the first said on period, and means operably responsive to the heating of the Work for initiating each off period.

4. In a control system for causing now of current to a load circuit from a source of alternating current during successive on periods and for preventing such ow during successive off periods, said periods each comprising a plurality of cycles of the source and said flow of current serving to heat work, the combination of electric discharge means actuable to pass current during each half cycle of each said on period, means for variably controlling the portion of each such half cycle throughout which said discharge means passes current to thereby cause the current so passed to gradually increase from a starting value to a maximum value and to thereafter decrease to a minimum value, means for preventing said current flow during said ofi periods, control means for initiating the iirst said on period, impulsing means operable periodically during each on period for controlling said second-mentioned means, and means operably responsive to the heating of the work for initiating each 01T period.

5. In a control system for causing flow of current to a load circuit from a source of alternating current during successive on periods and for preventing such flow during successive 01T periods, said periods each comprising a plurality of cycles of the source and said ow of current serving to heat Work, the combination of electric discharge means actuable to pass current during each half cycle of each said on period, means for variably controlling the portion of each such half cycle throughout which said discharge means passes current to thereby cause the current so passed to gradually increase from a starting value to a maximum value and to thereafter decrease to a minimum value, means for preventing said current flow during said on periods, control means for initiating the iirst said on period, impulsing means operable periodically during each on period for controlling said second-mentioned means, and means operably responsive to the heating of the work for initiating each on period, said last-mentioned means serving to delay the beginning of each off period until the current has fallen to said minimum value.

6. In a control system for controlling flow of current between a Work circuit and a source of periodic current, said current serving to heat work, electric valve means for controlling said flow, control means for actuating said valve means to enable said flow during a succession of periods of said source, varying means for causing the portions of the said periods throughout which said flow occurs to vary in accordance with a predetermined pattern, and means operably responsive to the heating of the Work for determining the number of periods te be included in said succession.

7. In a control system for controlling flow of current between a work circuit and a source of periodic current, said current serving to heat work, electric valve means for controlling said now, control means for actuating said valve means to enable said flow during a succession of periods of said source, varying means for causing the portions of the said periods throughout which said flow occurs to vary in accordance with a predetermined pattern, one of said varying and control means being operably responsive to the other, and means operably responsive to the heating of the work for determining the number of periods to be included in said succession.

8. In a control system for controlling ilow of current between a work circuit and a source of periodic current, electric valve means for controlling said flow, control means for actuating said valve means to enable said ow during a succession of periods of said source, varying means for causing the portions of the said periods throughout which said iiow occurs to vary in accordance with a predetermined pattern, timing means for operating said varying means, and means operated as a consequence of the flow of said current for additionally operating said varying means to alter said pattern.

9. In a control system for controlling ow of current between a Work circuit and a source of periodic current, said ilow serving to heat work, electric valve means for controlling said ilow, control means for actuating said Valve means to enable said ilow during a succession of periods of said source, varying means for causing the portions of the said periods throughout which said ow occurs to Vary in accordance with a predetermined pattern, timing means for controlling said varying means, and means operably responsive to the heating of the work for additionally controlling said varying means to thereby alter said pattern.

10. In a control system for controlling ilow of current between a. work circuit having an impedance which is subject to variation and a source of periodic current, normally non-conductive electric valve means for controlling said flow, means including a control valve for rendering said valve means conductive at a controlled point in each period of the source during which said current is to flow, the conductivity of said control valve serving to determine said point and said conductivity 'being determined by the potential impressed between a principal electrode and a control electrode with which said control valve is provided, means for impressing a predetermined potential between said electrodes, and

additional means responsive to an electrical characteristic of the work circuit for impressing an additional potential between said electrodes so as to maintain the average current in said work circuit at a, value corresponding to the setting of said starting means independently of variations in said impedance.

11. In a control system for controlling the ow of current between a load circuit and a source of alternating current, the combination of control means for causing current flow to said load circuit during each of a succession of on periods each comprising a controlled plurality of half cycles of said source and for causing the current during each on period to gradually increase from a starting value to a maximum value, and additional control means for causing current to gradually decrease at the conclusion of each on period from said maximum value to a minimum value and for preventing said cur rent flow during oir periods intervening between said on periods, each orf period comprising a controlled plurality 0f half cycles of said source, certain of said control means being operable as a consequence of manual control and others of said controls being operable as a consequence of the flow of current to said load circuit.

FRED H. JOHNSON. CHESTER F. LEATHERS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,314,691 Dawson et al Mar. 23, 1943 2,303,453 Gulliksen I- Dec. 1, 1942 2,250,207 Schneider July 22, 1941 2,113,220 Power c Apr. 5, 1938 2,175,920 Schnarz Oct. 10, 1939 2,327,268 Jenks Aug. 17, 1943 2,202,205 Howe May 28, 1940 2,166,305 Levoy, Jr., et al July 18, 1939 2,275,948 Dawson Mar. 10, 1942 2,361,846 Garman Oct. 3-1, 1944 FOREIGN PATENTS Number Country Date 117,852 Australia Dec. 16, 1943 

